A wavefront measuring device is known from DE 10 2005 026 628 A1.
A wavefront measuring device is used in a projection exposure apparatus for microlithography. Microlithography is one of the central technologies in semiconductor and microsystems engineering and serves for producing integrated circuits, semiconductor components and further electronic products. The basic concept of microlithography consists in transferring predefined structures to a substrate, for example a silicon wafer, via one or a plurality of exposure processes. The predefined structures typically contain micro- and/or nanostructures, formed on a reticle (also known as a photomask or mask). The substrate (wafer) is coated with a light-sensitive material (photoresist). During exposure, the exposure light is guided to the projection lens via the reticle, wherein the exposure light, after passing through the projection lens, finally reaches the substrate, where it acts on the light-sensitive material. In the subsequent development step, the substrate is treated with a solvent, such that, after the treatment, only the regions of the substrate surface which correspond to the predefined structures of the reticle are covered by the light-sensitive material; or, conversely, such that such regions become free.
Finally, the predefined structures of the reticle are transferred to the substrate in an etching step in which photoresist-free regions of the substrate surface are removed by an etching solution.
In semiconductor engineering it is desirable to realize structures having the smallest possible dimensions, in order to increase the number of circuits and/or semiconductor components that can be integrated on an area unit and thus to increase the performance of the semiconductor components. The structure size that can be realized microlithographically is directly dependent on the resolution capability of the projection lens, wherein the resolution capability of the projection lens is inversely proportional to the wavelength of the exposure light. Therefore, it is advantageous to use electromagnetic radiation from the short-wave range of the optical spectrum as exposure light. Nowadays it is possible to use ultraviolets (UV) light, in particular vacuum ultraviolet (VUV) light, as exposure light having a wavelength of 193 nm. The prior art also discloses microlithographic systems which use extreme ultraviolet (EUV) light having a wavelength of approximately 7 nm or 13.5 nm as exposure light. It is noted that the term “microlithography” should be understood broadly and generally relates not only to structure sizes in the range of less than 1 mm, but also in the range of less than 1 μm. In particular, microlithography even encompasses structure sizes in the nanometres range.
However, the use of UV, VUV and EUV light often leads to optical imaging aberrations, which are attributable for example to heating of the optical elements in the projection lens. The heating is therefore associated with the fact that the photonic energy of the exposure light is inversely proportional to the wavelength thereof. Consequently, optical elements are loaded by high heat input when light having short wavelengths is used, which leads to the impairment of the optical properties of the optical element, for example of the refractive indices, the reflection coefficients or the transmission coefficients of optical elements. The imaging aberrations caused thereby include monochromatic imaging aberrations such as spherical aberrations, astigmatism, coma, image field curvature and distortion. Chromatic imaging aberrations, such as transverse chromatic aberrations and longitudinal chromatic aberrations, can also arise as a result of such overheating.
In microlithographic processes, in order to increase the resolution it is often desirable for a plurality of successive exposure processes to be carried out. In this case, for the exposure quality it is crucial for the exposed structures of the successive exposure processes to be aligned highly accurately with respect to one another. This is accompanied by an increase in the desired accuracy behavior of the lateral structure positioning.
Furthermore, it is of importance to align the image plane of the projection lens with the photoresist as accurately as possible, in order to counteract telecentricity errors, for example, in which the exposure location varies in the light propagation direction. Telecentricity errors unavoidably lead to lateral image position variations that have an adverse effect on the exposure accuracy.
In addition to the causes mentioned above, unevennesses of the semiconductor substrate surface coated with the photoresist also lead to impairments of the imaging quality. This usually involves photoresist applied by spin coating, which has a non-uniform thickness owing to the application process. In order to compensate for the influences of such unevennesses, an accurate adaptation of the focal position of the projection lens to the substrate surface is desirable.
Against this background, optical wavefront manipulators are used to manipulate wavefronts of the exposure light and thereby to effect a correction of the aberrations. By way of example, an optical wavefront manipulator is known from DE 10 2013 204 391 B3, the optical wavefront manipulator having a manipulator surface whose surface shape and/or refractive index distribution are/is reversibly variable. A wavefront manipulation for dynamically influencing the wavefront of the exposure light rays is thus possible. A further example of an optical wavefront manipulator includes a plurality of movable positive-negative aspheres. The prior art furthermore discloses optical wavefront manipulators which have a facet mirror including a plurality of mirror facets which in each case are movable by an actuator system along at least one spatial direction and/or are tiltable about at least one axis.
In order that the optical wavefront manipulator functions with the high reliability for microlithography, a reliable measurement of wavefronts is of great importance. On the basis of the measured wavefront, the optical wavefront manipulator is correspondingly set in order to realize desired aberration corrections. The prior art discloses an interferometric wavefront measuring method that is suitable for determining telecentricity, distortion, coma and/or image shell aberrations. Moreover, it is known that a Moiré grating arrangement of the type mentioned in the introduction can be used for determining telecentricity errors.